Description: Here they have one of the marvels of the Northern World, and what do they say of it? Caves, they say! Caves! Holes to fly to in time of war, to store fodder in! My good Legolas, do you know that the caverns of Helm's Deep are vast and beautiful?

'And, Legolas, when the torches are kindled and men walk on the sandy floors under the echoing domes, ah! then, Legolas, gems and crystals and veins of precious ore glint in the polished walls; and the light glows through folded marbles, shell-like, translucent as the living hands of Queen Galadriel.

There are columns of white and saffron and dawn-rose, Legolas, fluted and twisted into dreamlike forms; they spring up from many-coloured floors to meet the glistening pendants of the roof: wings, ropes, curtains fine as frozen clouds; spears, banners, pinnacles of suspended palaces! Still lakes mirror them: a glimmering world looks up from dark pools covered with clear glass; cities. such as the mind of Durin could scarce have imagined in his sleep, stretch on through avenues and pillared courts, on into the dark recesses where no light can come. And plink! a silver drop falls, and the round wrinkles in the glass make all the towers bend and waver like weeds and corals in a grotto of the sea. Then evening comes: they fade and twinkle out; the torches pass on into another chamber and another dream. There is chamber after chamber, Legolas; hall opening out of hall, dome after dome, stair beyond stair; and still the winding paths lead on into the mountains' heart. Caves! The Caverns of Helm's Deep! Happy was the chance that drove me there! It makes me weep to leave them.'

TTT, Book III, Ch 8, The Road to Isengard

Etymology

aglar 'glory, brilliance' in Dagor Aglareb, Aglarond. The form in Quenya, alkar, has transposition of the consonants: to Sindarinaglareb corresponds Alkarinquë. The root is kal- 'shine' ...

rond meant a vaulted or arched roof, or a large hall or chamber so roofed; so Nargothrond..., Hadhodrond, Aglarond. It could be applied to the heavens, hence the name Elrond 'star-dome'.